so simple combiner,  make hist.
modify .gitignore.
combine input keys, make histogram.
change histogram keys to sym from string.
add watcher. - periodically flush hist data.
update configure method, and this method test.
can stripp tag, and add tag prefix.
tidy up.
add bench program. forked from https://github.com/frsyuki/fluent-benchmark.
modified README.